Michael Ryan, WHO's emergencies chief, made the announcement at a news conference in Geneva.

The Ebola outbreak that was declared in eastern Congo in August is already the second deadliest outbreak in history, and efforts to control it have been complicated by a volatile security situation and deep community mistrust.

Ebola treatment centers have come under repeated attack, and a Cameroonian epidemiologist working with WHO was killed last month.

Ryan says another attempted attack was made on the treatment center in Butembo city on Thursday but was repelled by security personnel. No one was injured.

VOA